67 Hood Tach install
 

Hi,
I did a search and cant find the answer im looking for. Maybe I missed it or maybe my situation is unique.
I had an older repro hood tach on my 67 GTO. I was cleaning it and broke the needle so I purchased a new hood tach from our sponsor.

I plugged the new tach into the existing plug where the old tach was plugged in and the new tach lights up real nice and when I turn on the car the needle goes all the way to the right then back to zero and then wont move. I was assuming the new tach would use the same wiring as the old one?

I have an MSD6AL #6420 and a MSD tach adapter PN 8920. I did not change anything, I just simply plugged the new tach into the old plug that was working fine.

Any suggestions? I'm very electrical challenged and that's why I was hoping I could just plug the new one into the old plug.
Thank you!

My repro tach does the same power up sequence: travels to full scale & back to zero. But works ok. I would suspect wiring to the tach adapter or the adapter itself.

Thanks for the suggestion Geoff. But the old tach works fine when I plug it in (just a broken needle) so that's why im confused as to why the new one wont work using the same wiring.
Thanks!

The hood tach as a reproduction part has been made by over 1/2 dozen different suppliers going back over a time span of at least the past 25 or 30 years. You do not say nor is anything labeled or marked as far as how long your old tach has been on the car or where it came from.

At this point there likely is no way to determine that anyway. Each supplier had their own way of making them as far as the circuit board and meter construction goes and none of the new ones are anything like the ones from even as "new" as 10 years ago. That said the modern one that I think you have now goes thru a "boot startup" every time you turn on your key. Your new tach needs at least 3 or perhaps 4 wires of electrical contact to work. (Some reproduction hood tachs are wired to be "lights on" at all times) where as the Some of the older reproductions had a separate grey light wire for night time driving. The original GM tach also had a separate grey wire.

Your new tach needs 12 volts of clean battery power, via the fuse block; thru a one or one & 1/2 amp fuse, when the key is turned on. It also needs ground. Last we need a wire from the new tach directly to the RPM signal output wire on the side of your red MSD6A module. I would NOT WIRE YOUR RPM seeker wire to the "-" terminal of the coil. I do not believe that you need any type of band-aid or adapter device with this new tach. It is made to work off of most any ignition system as long as it it correctly wired. Go to the MSD website and download the instructions for how to wire an accessory tach onto your MSD module.

It's pretty simple.

NOTE: The wire that is looking for the RPM signal from your engine needs to be fused. Same as the power wire. I recommend a one or one & 1/2 amp in-line glass fuse. It needs to be in one of those spring loaded plastic twist-holders made as part of the wire.

One thing about these new tachometer circuit boards is that they are not built to be very "forgiving." If you wire one incorrectly, or you wire one up without a ground, or you omit to use fuses in the power wire and the RPM seeker wire, the circuit board could be damaged. As far as I know none of the modern reproduction tachometers are made to be field serviceable. In other words they either work or they do not. There are no circuit diagrams available on any of that kind of stuff.
Be warned; if you cook one of those you get to buy another tach.

Thanks Peter.
Ill wire it up using all the new wires.
The instructions say the black with purple/pink stripe (it has an in-line fuse) goes to the negative side of coil but ill attach it to the port in the MSD6AL?
Solid black is a ground.
Brown wire is supposed to connect (it has a plug looking thing at the end) to the accessory terminal on the fuse box. I don't know what this is?
Does this sound right?
Thanks again!

Hi Mike, I would get someone over there to help you. A person who has a mechanical or electrical background. The instructions that come with that tach, they have no idea the vast array of modern aftermarket parts out there, as far as aftermarket and/or high-performance electronic ignition systems. The MSD boxes are very reliable. The MSD people have a 1-800 # as well as on-line support in the form of PDFs directions for all their MSD boxes. The coil wire negative RPM is for stock points ignition, that or a Pertronix One or Two. If you wire the tach up incorrectly it could damage the circuit board!!!!! There is an intense voltage spike at the "-" terminal of the coil on any low ohm Coil and or MSD controlled spark ignition system. That is why the MSD people give you a safe pre-amp RPM wire (or tab) that exits out the side of their red module. They are well aware that people would want to have a tachometer in these cars and the output RPM signal from the module is purpose made just for these cases.

I would take your car to a restoration shop that works on GTOs. The fuse block has extra tabs built-in for both battery power and Key on + power. If you wire that up wrong, it would be bad for your tach!!!